1. Field of the Invention
The present invention relates to a spread illuminating apparatus suitable for use as backlight unit of a liquid crystal display device.
2. Description of the Related Art
A liquid crystal display (LCD) device is used extensively as a display device for various electronic devices and recently is increasingly superseding a cathode ray tube (CRT) also in a relatively large display device as typified by, for example, a personal computer and a TV set.
For such a large LCD device, a direct light type backlight has been conventionally predominant which is structured such that a plurality of cold cathode lamps (hereinafter referred to as CCFLs as appropriate) as light sources are disposed immediately beneath an LCD panel, and in which light is condensed by an optical sheet and light intensity is uniformed by a diffuser sheet (refer to, for example, Japanese Patent No. 3653274).
Recently, various approaches to be described below are attempted for improving picture image quality and enhancing performance in an LCD device, and accordingly a backlight is becoming required to offer less conventional features.
For example, as a means of solving the visibility issue of movie display on an LCD panel, what is called a “pseudo impulse driving method” is established, in which a black display period is forcibly inserted between every two adjacent frame periods of a picture image display, whereby the pixel brightness response of the LCD panel is brought close to the impulse response.
In the pseudo impulse driving method, it is proposed that the black display period be produced by turning off the backlight, in which case the backlight is required to be capable of turning on and off rapidly. Another proposed method to produce the black display period is to input a black write signal in a picture image signal per frame period, also in which case the backlight is preferably turned on and off rapidly in synchronization with the black write signal in order to achieve an efficient pseudo pulse impulse driving.
Especially, in case of an LCD panel with a rapid response, a method is proposed in which a black write signal is inserted in a picture image signal per scanning line at a predetermined timing, whereby a black region is displayed at an area of a screen composed of a plurality of successive scanning lines, and at the same time the position of the black region displayed is shifted in synchronization with the scanning of the picture image (hereinafter this method is referred to as “black insertion driving method” as appropriate). In this case, it is required that only an area of the entire luminous region of the backlight located immediately beneath the black region be selectively turned off in synchronization with the shifting of the black region.
Also, what is called an “area control method” is proposed, in which the lighting condition of the backlight is partly controlled for enhancing the contrast of a picture image and reducing the power consumption. In this method, the backlight is driven such that when a black region which does not need illumination is present in a picture image as a display tone factor, the area of the luminous region located immediately beneath the black region is selectively turned off.
Unlike the above-described black region (black display period) forcibly inserted for improving the movie display performance, the black region working as a display tone factor for picture image is caused to vary in shape and size on the screen. Consequently, in order to achieve an effective area control, the areas of the luminous region of the backlight, which are adapted to turn on and off independently of one another, must be configured sufficiently minute and precise.
Conventional backlights, however, have the following difficulties in keeping up with the aforementioned improvement of image quality and enhancement of performance.
In a direct light type backlight with CCFLs, since a CCFL has a low response speed, and since the number of CCFLs provided in a backlight is usually restricted, commonly it is difficult to achieve a backlight to suitably match a pseudo impulse driving (especially, the above-described black insertion driving method).
Also, in a direct light type backlight with CCFLs, the luminous region cannot be divided into individual areas in the direction orthogonal to the length of the CCFLs because of its structure, and therefore the areas which are allowed to be area-controlled are forced to be very inadequately sized and shaped.
Meanwhile, recently, a direct light type backlight is proposed which employs a number of red (R), green (G) and blue (B) light emitting diodes (LEDs) as light sources, in place of the CCFLs, arranged immediately beneath an LCD panel, wherein a white light is produced by mixing lights from these LEDs. Such a backlight can evade the above-described problems with respect to the pseudo impulse driving and the area control.
However, the photoelectric conversion efficiency is low in achieving a white light by mixing lights from red, green and blue LEDs, thus raising problems with power consumption, heat value from light sources, cost, and the like for achieving a white light with an adequate brightness. For solving the problems about power consumption, heat value from a light source, and cost, the present inventors have looked at a quasi-white LED in which a quasi-white light is produced by transmitting a light of a blue LED through a luminescent layer containing yttrium aluminum garnet (YAG) particles, and have come up with a proposal to incorporate the quasi-white LED in what is called an edge light type backlight, which includes a light conductor plate having a major surface as a light outlet surface and a side surface at which a light source is disposed.
The edge light type backlight, however, has a peculiar problem that light leakage occurs such that a ball-shaped light called “hot spot” leaks principally at an LED, and another problem that what is called a “stray light” to cause an intense emission line is generated at an area of the light conductor plate close to the light inlet surface, which degrades the quality of the light of the backlight.
Such a stray light problem can be overcome for an LCD panel with a relatively small display area, for example, a mobile phone LCD panel, in such a manner that the area significantly suffering from the stray light is defined as a non-effective area to be located outside the display area of the LCD, whereby the brightness can be kept uniform across the display area of the LCD. This solution, however, cannot be applied when a plurality of lighting units each constituted by an edge light type backlight are arranged two dimensionally thus forming a multiple panel type backlight to cover a large LCD panel. That is to say, it is not possible for the non-effective areas of all the lighting units to be located outside the display area of the LCD.
FIGS. 6A and 6B are respectively a schematic perspective view and an exploded perspective view of a conventional multiple panel type backlight 10 which includes a plurality (36 in the figures) of lighting units 12, wherein a plurality of strip-shaped cover films 18 (constituted by light shielding films or light reflecting films) are placed so as to cover the areas positioned above light sources (LEDs) 14 and light inlet surfaces of light conductor plates 16, thereby eliminating the influence of the stray light as much as possible. The multiple panel type backlight 10 further includes a rectangular light reflecting film 20 to entirely cover the lower surfaces of all the lighting units 12.
The technique of using light shielding films for the cover films 18 is for lowering the brightness of stray light by absorbing light energy, and therefore the entire brightness of the multiple panel backlight 10 is forced to be lowered. So, the brightness of the LED 14 must be increased in order to provide a sufficient brightness for the multiple panel backlight 10, which results in an increased power consumption. On the other hand, the technique of using light reflecting films for the cover films 18 is to reflect stray light back into the light conductor plate to thereby make an effective use of the light. The utilization efficiency, however, is low, and the brightness of the LED 14 must be increased in order to provide a sufficient brightness for the multiple panel backlight 10, thus inevitably increasing power consumption. And, since both the light shielding film and the light reflecting film used as the cover film 18 do not allow transmission of light, the very existence of the cover film 18 develops a dark area thus deteriorating the brightness uniformity.